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Bacteria and viruses are behind most of the diseases we get hit by. It's nothing personal, it's just what they've got to do to survive.

Viruses are simple creatures. So simple they don't eat, drink or breed — they're just a bunch of genes dressed up in a protein coat. They get by by invading living cells (like ours), hacking their software (the DNA) and turning them into their own personal virus-making factories. The by-products of this new line of work generally aren't good for the host cell, and often cause it to burst, spilling thousands of the freshly minted virus particles to find other cells to invade and enslave.

Bacteria are a whole lot more civilised in comparison. For starters, they're alive. Each bacterium is one very basic single cell. Its life is a quest for a warm, wet spot with lots of food where it can eat and do what passes for breeding in bacterial circles. And from where bacteria sit, we are one big skinful of warm, wet food, there for the ingesting.

Not that we're an easy catch — as far as fortresses go, the human body is Edinburgh Castle wrapped in Kevlar. Our skin is a great barrier against microbes, so their only way in is via one of our mucus-lined entries or exits, or through some cut or bite. And once inside there's our entire immune system to contend with — when it comes to paranoid surveillance for intruders and 'eat first, question later', it has no equal.

In fact, most of the symptoms that we associate with being sick — fever, tiredness, redness and general misery — aren't caused by the bug we've caught, they're what happens when our immune system goes into attack mode. But before our immune system kicks into gear to wipe them out, these bugs have a few chemical tricks that make their presence felt.

The bacterial arsenal

Of the millions of bacterial species on the planet, only about 540 have got what it takes to cause disease in humans. But man, do they do it well.

Whether they sneak in through a bit of broken skin, or hitch a ride on some dodgy leftovers, pathogenic (disease causing) bacteria usually make their presence felt by throwing a bit of poison our way.

Some bacterial toxins work like classic poisons or drugs — they're just the right shape to fit a receptor molecule poking out of the surface of some of our cells. When they find the right kind of cell, the toxin binds to the receptor and cranks up the activity of an enzyme inside. That's how Escherichia coli (E. coli) causes diarrhoea (one good reason to not drink untreated water contaminated with faeces). The E. coli toxin binds to a receptor molecule on the cells lining your intestine, which activates an enzyme inside the cells. Suddenly, those cells aren't just gently oozing mucus into your intestine — they're pumping salty water in there. The more E. coli you swallowed, the more toxin and the more water. Goodbye firm stool, hello poo soup.

Other bacteria achieve a similar effect by literally punching holes in the cells lining our gut. Eating undercooked meat is a good way of feeling the effect. The molecules of toxin produced by Clostridium perfringens gang up like a circle of skydivers and literally punch a hole in the cell membrane, which not surprisingly leads to leaky, dying cells and plenty of liquid in the intestine.

And C. perfringens isn't a one-toxin pony. Another of its toxins doesn't settle for controlling the activity of enzymes in our cells — it acts like an enzyme itself. It's called alpha toxin, and it breaks down the fats in cell membranes, producing lots of gas. The result is gas gangrene. Nasty stuff.

If you're a fan of salty or sweet food that hasn't been stored properly, Staphylococcus can not only give you diarrhoea, it can add a top-end extra action too. The receptor that the staphylococci toxin binds to sends a message straight to the medullary vomiting centre in your brain. (It's the multi-coloured one in MRI scans. Not). Very soon the medulla shoots signals to your diaphragm and abdominal muscles to give your stomach a good old squeeze, and you'll swear off pork crackling chips for days.

Bacterial toxins can only do so much damage while they're in our guts. It's when they get through to our bloodstream and the toxin spreads around that their effects really take off.

Botulism is a classic example. The botulism toxin is best known for removing all signs of facial expression in deluded westerners. It works by blocking the action of one of our main neurotransmitters, acetylcholine — the one that nerves use to tell muscles to contract. But when you eat the botox, or the bacteria that makes it, Clostridium botulinum, the toxin gets into your blood stream and wreaks havoc. It's no longer just shutting down facial muscles, it can paralyse everything from your intestine to your respiratory muscles. Next stop, artificial respirator — or death.

We don't just get bacterial infections from food of course — wounds and sex are a great source of them too. A close relative of the botox-making bacterium, Clostridium tetani also affects our muscles, but not by switching them off, by leaving them permanently on. It's called tetanus, and we get it when bacterial spores in dirt get into our bloodstream through a cut. Once inside, the tetanus toxin floats around until it finds a nerve. From there it travels to the spinal cord where it blocks the release of neurotransmitters that tell muscles to relax. With more and more of your muscles permanently switched to 'on', there's no relaxing, no movement and eventually no breathing.

While clostridium toxins are the most toxic known, they're not the only way bacteria can kill us. Septic shock can happen when bacteria get into our bloodstream and their toxins attack the cells that make up our blood vessels. Liquid starts leaking out of our capillaries so there's not enough blood pressure to deliver oxygen to cells and remove waste. It's called shock, and if it isn't stopped, our organs suffer — liver, kidneys, heart and brain can all shut down. Meningococcal disease is a well-known form of septic shock, caused by bacteria that normally lives in our throats.

Toxic shock syndrome is caused by particular strains of two bacteria that normally live on our skin — Staphylococcus aureus and Streptococcus pyogenes. These toxins don't just act on regular cells, they send immune cells into hyperdrive. The chemical signals the immune cells usually send out to kickstart inflammation are suddenly flooding your body. Fever, rash and fluid leaking out of capillaries can again lead to organ failure and death.

Not all bacteria rely on toxins for making their way through the giant food-fest that is our bodies. Syphilis is caused by a bacteria that is shaped like a corkscrew, moves like corkscrew, and literally corkscrews its way through blood vessels, nerves and brain. Double nasty. Condoms, people!

It's not a man-flu, it's a head-cold

Viruses are a bit less gung-ho about killing our cells. They need them intact so they can use the cell's machinery to make clones of themselves. The trouble is that making high doses of viruses involves storing high concentrations of viral proteins, and they have a tendency to blow holes in cell membranes.

But cells weren't born yesterday, and to get inside them a virus needs to have the right kind of protein poking out of it to act like a pass card to a cell's bouncer. And different types of cells have got different locks.

A lot of viruses get inside us through our airways. Influenza viruses and rhinoviruses (that cause colds) take hold by 'unlocking' the receptors on the mucus-producing cells in our noses and throats. The runny nose, temperature and all the rest are caused by chemical signals given off by our immune cells rushing in to clear them out.

Herpes simplex — the virus behind cold sores — comes in on saliva or infected moist breath and follows the usual hijack/blow-up strategy on the cells it infects. The swelling and blisters are caused by chemicals released from the burst cell, which activate our inflammatory response. But the herpes virus isn't wiped out by our immune cells — it sneaks into the nearest nerve and travels along it until it hits the spinal cord. It stays dormant there until stress, sunburn or the prospect of a really big date brings it back along the nerve to give us a nice festering lump once again.

But viruses can cause much more damage than an ugly blister. After getting into our bodies through the mucosal cells in our gut, poliovirus makes its way to the spinal cord too, but not to use as a hiding place. The damage it does when multiplying in those cells causes paralysis.

Ebola virus is one of about 30 viruses that can kill by causing viral hemorrhagic fever. These viruses attack cells in our blood vessels, literally making them leaky. With holes in our pipes, our blood pressure drops putting us in a state of shock. Like bacterial toxic shock, nutrients can't get to our cells and waste builds up, with organ failure the next result. If our immune systems can beat the viruses in time, we can survive. Ebola is so deadly because it can kill within five days, while our immune system takes a week to swing into top gear.

Other viruses kill much more slowly by turning the cell they invaded into a tumour cell. The viruses behind genital warts (Human papilloma virus), glandular fever (Epstein-Barr virus) and hepatitis B and C viruses can all change the cellular DNA so those cells forget how to stop dividing. Uncontrolled cell growth is what cancer is all about — it causes the lumps that interfere with our regular functioning. About 12 per cent of human cancers are caused by viruses messing with the genes in the cells they infect.

With all those bugs out there it's a wonder we've lasted as long as we have. But most of them are no match for the search and destroy masters that make up our immune system. Between our white blood cells, and a host of chemical signals, those poor little bugs hardly stand a chance.